Sulfur- and phosphorus-containing additives such as zinc dialkyldithiophosphates, with excellent anti-wear and anti-oxidation properties have been used as substantially essential additives for conventional lubricating oils, particularly those for internal combustion engines. However, the lubricating oils have been strongly required to contain more less sulfur, phosphorus and ash as a result of necessity to alleviate harmful influences on exhaust-gas purifying catalysts such as ternary catalysts, oxidation catalysts and NOx adsorbers, or exhaust-gas after-treatment devices such as diesel particulate filters (DPF), which are equipped in internal combustion engines in conformity with recent environment-related issues.
Examples of low phosphorus or phosphorus free oils which have been disclosed so far include those described in Japanese Patent Laid-Open Publication Nos. 62-253691, 1-500912, 6-41568, 63-304095, 63-304096, 52-704, 62-243692, 62-501917, 62-501572, and 2000-63862. Examples of low ash oils include those described in Japanese Patent Laid-Open Publication Nos. 8-48989, 8-253782, 9-111275, and 2000-256690. However, when these known oils contain decreased or no zinc dithiophosphate, they need to be blended with a sulfur-containing compound for maintaining their anti-wear properties, while the low ash oils contain zinc dithiophosphate necessarily. So far there have been discovered few lubricating oils having excellent anti-wear properties and decreased in sulfur, phosphorus, and ash contents.
The present inventor found that low sulfur lubricating oil compositions containing a phosphorus-containing compound such as zinc mono- or di-alkylphosphtes, zinc dialkylmonothiophosphates or phosphoric acid triesters exhibited more excellent low friction properties, high temperature detergency, oxidation stability, and base number retention properties while maintaining anti-wear properties, than those containing zinc dithiophosphate only and have filed patent applications for such low sulfur lubricating oil compositions (Japanese Patent Application Nos. 2002-015351, 2001-315941, 2002-086145, 2002-086146, 2002-086147, 2002-191090, 2002-191091, and 2002-191092). However, in the case of using a salicylate-based detergent whose metal ratio is 5 or less, particularly that whose metal ratio is adjusted to be 3 or less, as a metallic detergent, a composition can be obtained which is extremely excellent in low friction properties, high temperature detergency, oxidation stability, and base number retention properties. However, it was found out that when the content of a sulfur- and phosphorus-containing anti-wear agent such as zinc dithiophosphate is decreased or a sulfur-free phosphorus-containing anti-wear agent is used, for decreasing the sulfur content, the resulting composition would not be able to exhibit sufficiently anti-wear properties for the valve train of an internal combustion engine, such as anti-scuffing properties for rocker arm pads and anti-wear properties for cams.
The present invention was made in view of the foregoing circumstances and intends to provide a long drain type low sulfur lubricating oil composition containing a salicylate-based detergent and having excellent anti-wear properties.